1. Technical Field of the Invention
The present invention relates generally to an engine start system for use in an idle stop system for automotive vehicles (also called an automatic engine stop/restart system) which works to stop an automotive engine automatically, for example, when the vehicle has stopped at an intersection or due to a traffic jam and then restart the engine when the vehicle driver performs a given starting operation (e.g., upon release of the driver's foot from the brake pedal).
2. Background Art
There are known automotive idle stop systems designed to stop the engine automatically when an output of the engine is required not to be produced and restart the engine automatically when the engine is requested to produce the output. In such systems, when a vehicle driver performs a given restarting operation after the engine is requested to stop, an engine restart request is produced to crank the engine. For example, Japanese Patent First Publication No. 2003-301765 discloses the above type of idle stop system.
Usually, starting of automotive engines is achieved by turning a ring gear joined to a crankshaft of the engine using an electronic starter to crank the engine. A pinion gear is brought into meshing engagement with the ring gear for rotating the ring gear. The pinion gear is driven by a starter motor. Before the engine is started, the pinion is usually not in mesh with the ring gear. When it is requested to start the engine, the pinion gear is pushed toward a given position and then engages the ring gear. The position at which the pinion gear is engagable or engages the ring gear will be referred to below as an engaged position. The position at which the pinion gear is not in engagement from the ring gear will be referred to below as a disengaged position.
Such engine restart control encounters a problem especially when the engine restart request is made after the engine stop request is made, but before the engine stops completely. Specifically, in such an event, restarting of the engine after it stops completely may lead to concern about the discomfort of the vehicle operator. Usually, a maximum length of time required by the engine to stop completely after being requested to be stopped is about one (1) second. However, when the accelerator pedal is depressed by the vehicle operator immediately after the engine is requested to stop, the engine may fail to restart quickly, thus causing the vehicle operator to feel uncomfortable.
It is, therefore, essential to restart the engine quickly without waiting until after the engine stops completely. Such engine restart control using the above conventional starter requires a determination of how to schedule the time when the pinion gear is to be moved from the disengaged position to the engaged position and the time when the electric power is to be supplied to the starter motor to rotate the pinion gear.
In recent years, there has been proposed to supply the electric power to the starter motor even when the engine is not requested to be restarted. For example, International Publication No. 2007/101770 A1 teaches an engine start system designed to supply the electric power to the starter motor without waiting for the engine restart request to rotate the pinion gear at the disengaged position every time the engine is requested to be stopped, monitor speeds of the pinion gear and the ring gear, and bring them into engagement with each other. In other words, such a system works to perform two tasks: one is to supply the electric power to the starter motor, and the other is to control the pushing of the pinion gear even if the engine restart request is not made.
The above system, however, poses the problem that the engagement of the pinion gear with the ring gear achieved each time the engine restart request is made generates mechanical noise with which vehicle occupants usually feel uncomfortable.
In order to alleviate the above gear engagement-caused noise, Published Japanese translation of International Patent Application No. 2008-510099 teaches controlling the flow of current through a solenoid actuator to move the pinion gear to the engaged position. This, however, requires a current control circuit such as a chopper circuit, thus resulting in a complex structure of the system.
The system, as taught in either of the above publications, is designed to supply the electric power to the starter motor when the engine restart request is not made, thus resulting in an increase in consumption of power in the battery. This leads to a great concern about an increase in consumption of fuel in the automotive vehicle or a lack of electric power to be supplied to electric accessories when the engine is restarted.
In order to eliminate the above problem, Japanese Patent First Publication No. 2005-330813 teaches an automatic engine stop/restart system designed to supply the electric power to the starter motor only when the engine restart request has been made, monitor speeds of the pinion gear and the ring gear, and move the pinion gear to the engaged position. The system, however, has already supplied the electric power to the starter motor upon engagement of the pinion gear with the ring gear whenever the engine restart request is made. There is, therefore, still a problem of an increase in consumption of fuel in the vehicle. Additionally, it is also necessary to monitor the speeds of the pinion gear and the ring gear, thus resulting in a complex structure of the system.
When the engine is requested to be restarted immediately after the vehicle stops, it is, as described above, essential to restart the engine quickly without waiting until after the engine stops completely, in other words, while the engine is still decelerating. The automatic engine stop/start system, as taught in the above Japanese Patent First Publication No. 2005-330813, starts to supply the electric power to a shunt coil to rotate the pinion gear when the engine restart request is made while the speed of the engine is still decreasing, and brings the pinion gear into engagement with the ring gear at the time when the rotation of the pinion gear has been found to be synchronous with that of the ring gear.
Japanese Patent First Publication No. 2007-107527 teaches a starter control system designed to bring the pinion gear into engagement with the ring gear when the speed of the engine lies between preselected maximum and minimum values, and the engine is rotating in the same direction as a forward direction of the crankshaft of the engine.
The automatic engine stop/start system in the above Japanese Patent First Publication No. 2005-330813, however, needs to synchronize the speed of the pinion gear with that of the ring gear and thus may fail in such synchronization in a range where the speed of the engine is much low because the speed of the engine drops before the starter starts to rotate. Additionally, when the engine has undergone a fuel cut in response to an idle stop request, the speed of the engine usually drops rapidly and overshoots a zero point, so that the rotation of the engine is reversed. The engine then decreases in speed down to zero while continuing to oscillate in rotation in the normal direction and the reverse direction cyclically for a while. In a range where the engine continues to oscillate in rotation in the normal direction and the reverse direction cyclically (which will also be referred to below as an engine speed oscillating range), it is difficult to synchronize the speed of the pinion gear with that of the ring gear. An excessive mechanical impact will also be produced upon engagement of the pinion gear with the ring gear, which may cause the damage to the ring gear.
The starter control system in the above Japanese Patent First Publication No. 2007-107527 needs to measure the direction in which the engine is rotating in order to bring the pinion gear into engagement with the ring gear only when the direction of rotation of the engine agrees with the forward direction of the crankshaft. In other words, this system inhibits the pinion gear from being brought into engagement with the ring gear in a range where the engine is rotating in the reverse direction, thus resulting in a difficulty in achieving the engagement of the pinion gear with the ring gear in the engine speed oscillating range.